Butt contacts have many advantages and many limitations, e.g. as means for insertion of high current-carrying-capacity fuses into electric circuits.
As seen in considerable magnification, the cooperating surfaces, or overlapping surfaces, of a pair of engaging butt contacts are highly irregular arrays of projections. Current transfer from one of the engaging contacts is limited to the areas of these projections. These areas are but a very small fraction of the total area of overlap of a pair of mutually engaging butt contacts. It is thus apparent from the above that butt contacts are very inefficient in terms of area, area efficiency being the ratio of the sum of the areas of actual current-transfer to the total contact area, or that of contact overlaps.
Contact resistance at each current transferring projection depends upon a number of parameters and is difficult to determine with any degree of accuracy. Important parameters that determine the contact resistance at discrete spaced points of current transfer are contact pressure and hardness of the metal of which the contacts are made. The contact pressure may vary within wide limits and result in elastic deformation of some of the current transferring projections and in plastic deformation of others of these projections.
It is apparent from the above that the aggregate resistance of a pair of engaging butt contacts is roughly inversely proportional to the extent of their overlapping surfaces, the number of points where pressure is applied to the contacts, i.e. the number of clamping screws which exert pressure upon the contacts at spaced points thereof, and to the extent or magnitude of that pressure.
The cost of butt contacts are relatively small, and their resistance is relatively stable. Both these facts are significant advantages of this type of contacts.
The limitations of butt contacts are apparent from what has been said above. They require large areas of overlap which may result in highly undesirable bulk. They further require, when the current to be carried is high, a plurality of clamping screws which to tighten or untighten involves much time.
It is the principal object of this invention to provide compact contact clamping means capable of establishing high contact pressures, in particular to provide fuse holders for fuses having a high current-carrying capacity, but relatively small contact surfaces. In other words, the contact surfaces of fuses according to the present invention may be smaller than that of fuses having conventional butt contacts designed for a comparable current-carrying duty. Fuses embodying the present invention have screw means capable of establishing large contact pressures and capable of being rapidly tightened and untightened so as to minimize the downtime normally involved in the exchange of fuses.